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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: July 12, 2011                                          Kirei AB
                                                         January 8, 2011

  Using Secure DNS to Associate Certificates with Domain Names For TLS


   TLS and DTLS use certificates for authenticating the server.  Users
   want their applications to verify that the certificate provided by
   the TLS server is in fact associated with the domain name they
   expect.  Instead of trusting a certification authority to have made
   this association correctly, the user might instead trust the
   authoritative DNS server for the domain name to make that
   association.  This document describes how to use secure DNS to
   associate the TLS server's certificate with the the intended domain

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 12, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The first response from the server in TLS may contain a certificate.
   In order for the TLS client to authenticate that it is talking to the
   expected TLS server, the client must validate that this certificate
   is associated with the domain name used by the client to get to the
   server.  Currently, the client must extract the domain name from the
   certificate, must trust a trust anchor upon which the server's
   certificate is rooted, and must successfully validate the

   Some people want a different way to authenticate the association of
   the server's certificate with the intended domain name without
   trusting the CA.  Given that the DNS administrator for a domain name
   is authorized to give identifying information about the zone, it
   makes sense to allow that administrator to also make an authoritative
   binding between the domain name and a certificate that might be used
   by a host at that domain name.  The easiest way to do this is to use
   the DNS.

   This document applies to both TLS [RFC5246] and DTLS [4347bis].  In
   order to make the document more readable, it mostly only talks about
   "TLS", but in all cases, it means "TLS or DTLS".  This document only
   relates to securely associating certificates for TLS and DTLS with
   host names; other security protocols are handled in other documents.
   For example, keys for IPsec are covered in [RFC4025] and keys for SSH
   are covered in [RFC4255].

1.1.  Certificate Associations

   In this document, a certificate association is based on a
   cryptographic hash of a certificate (sometimes called a
   "fingerprint") or on the certificate itself.  For a fingerprint, a
   hash is taken of the binary, DER-encoded certificate, and that hash
   is the certificate association; the type of hash function used can be
   chosen by the DNS administrator.  When using the certificate itself
   in the certificate association, the entire certificate in the normal
   format is used.  This document also only applies to PKIX [RFC5280]

   Certificate associations are made between a certificate or the hash

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   of a certificate and a domain name.  Server software that is running
   TLS that is found at that domain name would use a certificate that
   has a certificate association given in the DNS, as described in this
   document.  A DNS query can return multiple certificate associations,
   such as in the case of different server software on a single host
   using different certificates (even if they are normally accessed with
   different host names), or in the case that a server is changing from
   one certificate to another.

1.2.  Securing Certificate Associations

   This document defines a secure method to associate the certificate
   that is obtained from the TLS server with a domain name using DNS
   protected by DNSSEC.  Because the certificate association was
   retrieved based on a DNS query, the domain name in the query is by
   definition associated with the certificate.

   DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
   [RFC4034], and [RFC4035]), uses cryptographic keys and digital
   signatures to provide authentication of DNS data.  Information
   retrieved from the DNS and that is validated using DNSSEC is thereby
   proved to be the authoritative data.  The DNSSEC signature MUST be
   validated on all responses in order to assure the proof of origin of
   the data.

   This document only relates to securely getting the DNS information
   for the certificate association using DNSSEC; other secure DNS
   mechanisms are out of scope.

1.3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   A note on terminology: Some people have said that this protocol is a
   form of "certificate exclusion".  This is true, but in a very unusual
   sense.  That is, a DNS reply that contains two of the certificate
   types defined here inherently excludes every other possible
   certificate in the universe other than those found with a pre-image
   attack against one of those two.  The certificate type defined here
   is better thought of as "enumeration" of a small number of
   certificate associations, not "exclusion" of a near-infinite number
   of other certificates.

   Some of the terminology in this draft may not match with the
   terminology used in RFC 5280.  This will be fixed in future versions
   of this draft, with help from the PKIX community.  In specific, we

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   need to say (in a PKIX-appropriate way) that when we say "valid up
   to" and "chains to", full RFC 5280 path processing including
   revocation status checking is intended.

2.  Getting TLS Certificate Associations from the DNS

   This document defines a new DNS resource record type, "TLSA".  A
   query on a domain name for the TLSA RR can return one or more records
   of the type TLSA.  The TLSA RRType is TBD.

   The format of the data in the resource record is a binary record with
   three values, which MUST be in the order defined here:

   o  A one-octet value, called "certificate type", specifying the
      provided association that will be used to match the target
      certificate.  The types defined are:

         1 -- Hash of an end-entity certificate

         2 -- Full end-entity certificate in DER encoding

         3 -- Hash of an certification authority's certificate

         4 -- Full certification authority's certificate in DER encoding

   o  A one-octet value, called "hash type", specifying the type of hash
      algorithm used for the certificate association.  This value has
      the same values as those of the TLS hash, as defined in the IANA
      registry titled "TLS HashAlgorithm Registry"
      (<http://www.iana.org/assignments/tls-parameters>).  For example,
      the value for the SHA-1 hash function is "2".  When no hashing is
      used (that is, in the certificate types where the full certificate
      is given), the hash type is 0.  Using the same hash algorithm as
      is used in the signature in the certificate will make it more
      likely that the TLS client will understand this TLSA data. [[
      Note: this is currently being discussed in the WG as issue #4, so
      it could change. ]]

   o  The bytes containing the certificate or the hash of the associated
      certificate (that is, the certificate or the hash of the
      certificate itself, not of the TLS ASN.1Cert object).

   Certificate types 1 through 4 explicitly only apply to PKIX-formatted
   certificates.  If TLS allows other formats later, or if extensions to
   this protocol are made that accept other formats for certificates,
   those certificates will need certificate types. [[ Later: maybe make
   yet-another-probably-never-used IANA registry for certificate types.

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2.1.  Making Certificate Associations

   The TLS client determines whether or not the certificate offered by
   the TLS server matches the certificate association in the TLSA
   resource record.  If the certificate from the TLS server matches, the
   TLS client accepts the certificate association.  Each certificate
   type has a different method for determining matching.

   For types 1 and 3, the hash used in the comparison is the hash type
   from the TLSA data.

   Types 1 (hash of an end-entity certificate) and 2 (full end-entity
   certificate) are matched against the first certificate offered by the
   TLS server.  For type 1, the certificate association is valid if the
   hash of the first certificate offered by the TLS server matches the
   value from the resource record.  For type 2, the certificate
   association is valid if the certificate in the TLSA data matches to
   the first certificate offered by TLS.

   Type 3 (hash of certification authority's certificate) can be used in
   one of two ways.  If the hash of any certificate past the first in
   the certificate bundle from TLS matches the value from the TLSA data,
   and the chain in the certificate bundle is valid up to that
   certificate, then the certificate association is valid.  Alternately,
   if the first certificate offered chains to a trust anchor, and the
   hash of that trust anchor matches the value from the TLSA data
   (assuming that the trust anchor is kept in certificate format), then
   the certificate association is valid.

   Type 4 (full certification authority's certificate) is used in
   chaining from the end-entity given in TLS.  The certificate
   association is valid if the first certificate in the certificate
   bundle can be validly chained to the certificate from the TLSA data
   (assuming that the trust anchor is kept in certificate format).

   [[ Need discussion of self-signed certificates being CA certificates.
   Need to be sure that this discussion uses correct PKIX terminology
   and is carefully explained. ]]

2.2.  Presentation Format

   The RDATA of the presentation format of the TLSA resource record
   consists of two numbers (certificate and hash type) followed by the
   bytes containing the certificate or the hash of the associated
   certificate itself, presented in hex.  An example of a hash of an
   end-entity certificate:

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      www.example.com. IN TLSA (
          1 2 e77b719d4e9c63c0b0a0333be0a4188e490b618e )

   The use of mnemonics instead of numbers is not allowed.

   [[ We could consider using Base64 instead of hex. ]]

2.3.  Wire Format

   [[ Need to do this, clearly. ]]

3.  Use of TLS Certificate Associations in TLS

   In order to use one or more TLS certificate associations described in
   this document obtained from the DNS, an application MUST assure that
   the certificates were obtained using DNS protected by DNSSEC.  TLSA
   records must only be trusted if they were obtained from a trusted
   source.  This could be a localhost DNS resolver answer with the AD
   bit set, an inline validating resolver library primed with the proper
   trust anchors, or obtained from a remote nameserver to which one has
   a secured channel of communication.

   If a certificate association contains a hash type that is not
   understood by the TLS client, that certificate association MUST be
   marked as unusable.

   An application that requests TLS certificate associations using the
   method described in this document obtains zero or more usable
   certificate associations.  If the application receives zero usable
   certificate associations, it processes TLS in the normal fashion.

   If a match between one of the certificate association(s) and the
   server's end entity certificate in TLS is found, the TLS client
   continues the TLS handshake.  If a match between the certificate
   association(s) and the server's end entity certificate in TLS is not
   found, the TLS client MUST abort the handshake with an
   "access_denied" error.

3.1.  Certificate Validation by TLS Clients When Using Certificate

   TLS client policy is deliberately not prescribed by this
   specification.  A client MAY choose to trust a DNSSEC-secured
   certificate association, depending on its local policy.

   [[ The preceding paragraph is probably wrong in the sense that it
   means that we now have no conformance requirements.  There is

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   probably no reason to even use this protocol unless you are going to
   fully trust the results.  The one exception that has been discussed
   is that you might want to use the TLSA data as a "second positive
   opinion", such as in a GUI or in logging.  Both of those seem fairly
   useless in the case of DNS resolution.  Thus, the above paragraph may
   be changed by the WG in a future version of this draft. ]]

3.1.1.  Use of Self-Signed Certificates

   One expected use case for this protocol is that some TLS servers will
   begin to use self-signed certificates in association with certificate
   associations.  A TLS client that is using this protocol needs to
   treat self-signed certificates as special, and thus SHOULD NOT
   attempt certificate validation on them.  (An exception to this rule
   would be clients that keep self-signed end entity certificates in its
   trust anchor store.)

3.1.2.  Ignorning Host Names in Self-Signed Certificates

   All data in a self-signed certificate other than the key itself can
   be ignored as untrusted unless a client validates the self-signed
   certificate to a trust anchor that is identical to the certificate.
   That means that the host name given in the self-signed certificate is
   meaningless, and that the only way to associate the public key in the
   certificate with the domain name is through the certificate
   association made in the DNS.

   If a TLS client fully trusts the association between a domain name
   and the certificate that was provided by the DNS, then that client
   MUST ignore the domain name that is given in the self-signed
   certificate.  That is, the certificate might contain a domain name
   that is different than the one that was used to get the TLSA data,
   but if the client is trusting the TLSA data, it doesn't matter what
   domain name is used in the certificate.  An expected use case for
   this protocol is to allow someone who controls the private key on a
   certificate to use that certificate for multiple TLS servers.  These
   servers might be on a single computer that has many domain names
   (such as a computer that is both a web host and a mail host, and is
   known by both "www.example.com" and "smtp.example.com"), or they
   might be on different computers (such as multiple computers that all
   respond IP addresses reachable as "www.example.com").

   [[ Add more about virtual hosting and SNI TLS extension. ]]

3.1.3.  Use of Local Trust Anchors

   Another expected use case for this protocol is that some TLS servers
   will use certificates that chain to a trust anchor that might not be

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   one that is trusted by the TLS client, such as a local certification
   authority (CA) that is administered by the organization that runs the
   TLS server; this is a likely use for certificate types 3 and 4.
   Because of this, a TLS client that is using this protocol that
   performs certificate validation on server certificates MAY have a
   method to communicate with the user that differentiates between
   validation failures that occur on certificates that have had secure
   certificate associations and those that have not.  If it does not
   have such a method of communication, the failure to validate SHOULD
   cause the same error as for any other certificate validation.

3.1.4.  Use of Additional Certificate Data

   Some TLS clients extract data from the certificate other than the key
   to show to the user; for example, most modern web browsers have the
   ability to show an extended validation (EV) name that is embedded in
   a certificate.  Because this data comes from a trusted third party
   and not the TLS server itself, TLS clients that extract additional
   information from TLS server certificates MUST validate those
   certificates in the normal fashion.

4.  IANA Considerations

   This document uses a new DNS RRType, TLSA, whose value is TBD.  A
   separate request for the RRType will be submitted to the expert
   reviewer, and future versions of this document will have that value
   instead of TBD.

5.  Security Considerations

   The security of the protocols described in this document relies on
   the security of DNSSEC as used by the client requesting A and TLSA

   A DNS administrator who goes rogue and changes both the A and TLSA
   records for a domain name can cause the user to go to an unauthorized
   server that will appear authorized, unless the client performs
   certificate validation and rejects the certificate.  That
   administrator could probably get a certificate issued anyway, so this
   is not an additional threat.

   The values in the TLSA data will be normally entered in the DNS
   through the same system used to enter A/AAAA records, and other DNS
   information for the host name.  If the authentication for changes to
   the host information is weak, an attacker can easily change any of
   this information.  Given that the TLSA data is not easily human-

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   readable, an attacker might change those records and A/AAAA records
   and not have the change be noticed if changes to a zone are only
   monitored visually.

   If the authentication mechanism for adding or changing TLSA data in a
   zone is weaker than the authentication mechanism for changing the
   A/AAAA records, a man-in-the-middle who can redirect traffic to their
   site may be able to impersonate the attacked host in TLS if they can
   use the weaker authentication mechanism.  A better design for
   authenticating DNS would be to have the same level of authentication
   used for all DNS additions and changes for a particular host.

   [[ Add discussion of the idea that TLSA makes things worse if an
   intermediate CA is compromised.  Need more from Stephen Farrell. ]]

   [[ Add discussion of length check to avoid potential issues with
   appended data.  Need more from Carl Wallace. ]]

6.  Acknowledgements

   Many of the ideas in this document have been discussed over many
   years.  More recently, the ideas have been discussed by the authors
   and others in a more focused fashion.  In particular, some of the
   ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
   Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley,
   Ilari Liusvaara, and Ondrej Sury.

7.  References

7.1.  Normative References

   [4347bis]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", draft-ietf-tls-rfc4347-bis (work in
              progress), July 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

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   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

7.2.  Informative References

   [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
              Material in DNS", RFC 4025, March 2005.

   [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
              Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
              January 2006.

Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se

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